Fas ligand (FasL) is produced as a membrane-bound and soluble protein and both forms are expressed within the immune privileged eye. In our previous studies we used an ocular tumor model to demonstrate that the different forms of FasL regulate innate immunity in the eye: (i) membrane FasL (mFasL) induces inflammation and terminates immune privilege, while (ii) soluble FasL (sFasL) prevents inflammation and maintains immune privilege. Therefore, given that FasL is constitutively expressed within the eye, and the membrane form of FasL is pro-inflammatory, we propose that in a normal eye either: (i) the pro-inflammatory function of mFasL is blocked, and/or (ii) FasL is expressed primarily in the soluble (anti-inflammatory) form. Our experimental data support the latter. Western blot analysis of FasL was performed on normal, FasL knockout, and chronically inflamed eyes. A high ratio of soluble (27 kDa) to membrane (38 kDa) FasL (10:1) was detected in normal eyes. Three additional bands (28-31 kDa) of modified sFasL were detected that were completely absent in the eyes of FasL knockout mice. The modified sFasL was unique to the eye and not found in other immune privileged sites, such as the testis. Finally, the 27kD and 31kD sFasL bands were completely absent from eyes that lacked immune privilege and displayed chronic inflammation secondary to pigment dispersion syndrome. We hypothesize that modified sFasL is expressed within the eye and plays a central role in maintaining immune privilege. This hypothesis will be tested in three Specific Aims (i) determine where sFasL is expressed in the eye and how it is modified, (ii) determine how modified sFasL stimulates innate immunity, and (iii) determine if modified sFasL controls immune privilege. We believe this study will advance not only our understanding of immune privilege, but help explain the controversy over the physiological role of FasL in transplants and tumors.